Bleaching cellulose with titanium compounds



Patented July 15, 1941 BIlEACEENG CELLU'LOSE WITH TITANIUM COMPOUNDS Alfred 0. Bragg, Valley Stream, N. Y., assignor to Diamond Alkali Company, Pittsburgh, Pin, a corporation Delaware No Drawing. Application June 28, 1940,

Serial No. 342,976

l7 illoims.

This invention relates to the bleaching of cellu= lose, and more particularly to improvements in processes of bleaching cellulose in which bleachmg is assisted or activated by a titenium compound in which the titanium has a valence less than four.

Although the invention is applicable generally to the bleaching of cellulose and cellulosic materials in their various forms, it will be described with particular reference to the bleaching of a process of bleaching. cellulose materials in accordence with which on alkaline aqueous suspension of the cellulose material containing an oxidizing bleaching agent is subjected to the action of a titanium compound in which the titanium has a valence of less than four. The titanium reagent used in practicing the invention of that patent apparently activates the bleaching function, and d major resultthereof is to reduce the time required to bleach to any particular standard of whiteness as compared with a process using similar reagents except for the titanium compound. Extended experience in the pi'octice of that patented process has demonstrated amply that although it is capable of satisfactorily accomplishing its stated objects, the results are erratic and not satisfactorily reproducible under all conditions of practice. Thus, while the process performs satisfactorily in bleaching as conducted commercially by some concerns, comparable results are not obtained when the process is applied to diflerent conditions used commercially by other cellulose bleachers. In other words, although excellent results are obtained at times or with some cellulosic materials, the results are less satisfactory at other times or with other celluloslc materials and for no apparent reason. The results eppenr to be erratic, more specifically, whenchlorine-com taining bleaching agents are used.

It is among the objects of this invention to provide 0. process of bleaching cellulose with chlorine-containing oxidizing bleaching agents anti titanium compounds in which the valence of titanium is less than, tour which is reliable, applicable generally to oxidizing bleaching pro- (ill. 8-105) ceduresinvolving the reagents named, does not interfere with existing practices, effects substantial economies in bleach requirements and in operating time, improves the yields and qualities of the beached products, is simple and easily practiced, and overcomes, at least in large part, the troubles referred to.

As described in the above-mentioned patent, various oxidizing bleachingagents may be used in the practice of the invention described. Thus,-

the oxidizing bleaching agent may be any of those available for such purposes, such as potassium permanganate, potassium dichromate, sodium perborate, hydrogen peroxide, or the like, all of which exert e pure oxidizing action. For most purposes, however, it is preferred to use the chlorine-containing bleaching agents of which the chlorites and hypochlorites are best known and used most widely, particularly those commonly' used in the art for this purpose and typifled by sodium hypochlorite and calcium hypochlorite. These chlorine-containing bleaching agents exert not only an oxidizing action, but also a chlcrinaitlng action. I have discovered, and it is upon this that the invention is predicated, that certain of the dimculties referred to above in the practice of the process of the aforesaid patent are due to this chlorinating function 01' the chlorine-containing oxidizing bleaching agents.

In the use of these chlorine-containing oxidizing bleaching agents the bleaching action is due, as stated, in part to the liberation of oxygen and, to a lesser degree, to the liberation of chlorine by the action of hydrochloric acid according to the formula:

the outset of the bleaching process. I have 'dis-. a covered that this is particularly the case wherealkali and titanium compound in which the titanium has a valence less than four are added at the beginning of the process because the additional sodium or calcium chloride due to the alkali addition accelerates the reactions.

I have discovered that the benefits of titanium compound in which the titanium has a valence of two or three are not realized fully itsuch compound is added to an aqueous suspension of impure cellulosic material containing alkali and hypochlorite bleaching agent capable of exerting a chlorinating action while the chlorinating action is going on. Stated otherwise, I have discovered that in the bleaching of cellulosic material with chlorine-containing oxidizing bleaching agents and titanium compounds in which the titanium has a valence less than four, more con-= sistent and reliable results, with other benefits, are to be had by deferring the addition of the titanium compound until after the chlorinating action has been completed or substantially completed.

In addition to greater consistency and relia bility of action, other advantages also flow from the practice of the present invention. Particularly, the amount of bleaching agent can be reduced substantially as compared with similar processes in which the titanium compound is not used, and with measurable improvement in brightness. in consequence of this degradation of the pulp is decreased and the yield of bleached cellulosic material is increased. Also, very real reduction in bleaching time is effected.

In the practice of the invention, therefore, the addition of the titanium compound is not made until chlorination ceases, and by having it present only during the purely oxidizing stage oi bleaching its characteristic benefits, as described in my aforesaid patent, are realized fully, whereas, on the other hand, they are more or less wasted if the titanium compound is present during the chlorination stage.

In my copendingapplication Serial No. 332,779, filed May 1, 1940, I have disclosed and claimed another factor which is generally useful with all oxidizing bleaching agents in securing improved results in themractice oi the aforesaid patented invention. It resides, essentially, in having iree oxygen, e. g., air, present in the suspension when the titanium compound is added and during its period of action. This may be accomplished by passing oxygen or air into the aqueous alkaline suspension of fiber and bleaching agent, and for most purposes this is accomplished suitably by agitating the furnish violently with air whereby suflicient oxygen will be taken up by the pulp suspension. Theexact amount of air necessarily provided in the pulp suspension will depend upon the amount of and the particular titanium compound which is to be used. I have i'ound, however, that with the amounts of the titanium compounds .used for these purposes it sufflces to effect saturation of the suspension with air, although preferably an excess oi oxygen is supplied by continuing agitation in air or blowing in.

of air during the treatment. I now believe that the requirement is to supply air in an amount such as to provide the oxygen needed to convert the titanium in the titanium reagent at least to the quadrivalent state, or a corresponding amount of pure oxygen.

Especially desirable results are to be had inacteristic of chlorinated products.

In the practice oi. the present invention, therefore, there is provided an aqueous suspension of the cellulosic material, for example pulp fiber.

and an oxidizing bleaching agent, suitably sodium or calcium hypochlorite, and the reaction is allowed to proceed until the chlorination 0! impurities is complete, or substantially so. The suspension is provided with the necessary amount of oxygen, either prior to, during, or after that stage, as by passing oxygen or air into the aqueous alkaline suspension of fiber and bleaching agent, preferably by agitating the furnish violently in contact with air whereby a sufficient amount of oxygen will be dissolved or otherwise taken up by the pulp.

As an example, reference may be made to the single stage hypochlorite bleaching of raw kratt pulp, which contains a substantial amount of material susceptible to chlorination. Upon the addition of the hypochlorite the pulp turns at once to a dark chocolate color which is char- The color lightens gradually until at the end of, say, fifteen or twenty minutes, it begins to assume a salmonyellow color. During this interval the action changes from one of substantially pure chlorination to one of substantially pure oxidation. The point at which chlorination ceases and oxidation begins can be found by pH and 2H measurements.

After chlorination has ended the titanium compound is added in accordance with this invention. Additional allrali over that oi. the hypoehlorite is necessary for the titanium compound to exert its action properly. Although the alkali may be added with thehypochlorite, it is preterable not to do so because the speed of chlorination is reduced with lessening of the great reduction in bleaching time attainable by this inven-,

tion. This is due to the fact that chlorination proceeds faster the lower the pH of the mixture, and the added alkali needed for the action 01' the titanium compound raises the pH of the suspension so far above that due to the hypochlorite alone as to reduce the rate of chlorination and increase the over-all bleaching time. Hence in the practice of this invention as applied to single stage bleaching processes it is preferred not to add the alkali until after chlorination has ended, and at the same time to add the air, and then to add the titanium compound.

In multi-stage processes, e. g. using elemental chlorine initially Iollowed by a chlorine-containing oxidizing bleaching agent, the chlorination will be completed in the first stage, and the alkali and titanium compound may then be added in later stages, for instance in the second stage. Here the hypochlorite can be added with the alkali and the titanium compound added after the necessary amount oi. air has been introduced into the pulp. Or, the hypochlorite, alkali and titanium compound may be added in rapid succession, with time between each addition for thorough mixing and for saturating the pulp with air.

Various titanium compounds in which the titanium has a valence less than four may be used as described in the aforesaid patent. The reagent may be titanium sulfate (TMSOOa). titanium dichloride ('IiCh), titanium hydroxide (Tl(OH)2), and the like, although the sulfate is preferred for many purposes. The inorganic titanium compounds are preferred, especially those soluble in water or acids, and they may be added in the dry state, in solution, or in suspension. Usually it is desirable to add this reagent as a solution, and'where the sulfate is used the solution should'be acid and air-free.

Alkali in addition to that carried by the hypochlorite may be supplied, todevelop the action of the titanium compound, by any of the wellknown alkali or alkaline earth hydroxides although caustic soda is preferred for most purposes. I now believe that for the best results the furnish should have a pH of at least about 11 when the titanium reagent is added.

In high density beaters, say beyond 6 or 7 per cent, agitation alone may not suffice. but up to that concentration ordinary operation of a beater affords satisfactory results without the introduction of extra air or oxygen.

In the tests of the invention the best results have been obtained by introducing the titanium compound slowly, preferably dropwise, into the agitated zone, most suitably into the region of highest oxygenconcentration.

As an example of the practice of the'invention and the benefits to be derived from it, reference will now be made to one test in which it was applied to a single stage hypochlorite bleach of hard Southern pine kraft stock. The pulp was mixed with calcium hypochlorite bleach liquor in an amount sufficient to provide 6.5 per cent of available chlorine based on the weight of the pulp. After 10 minutes the chlorination stage had ended, and lime was then added. After minutes more the titanium compound was added, the beater having been operated continuously to saturate the pulp with oxygen. Bleaching was continued to a totaltime of 3 hours and 45 minutes following which the bleached pulp was washed.

In comparison, a similar amount of the same pulp suspension was treated in accordance with regular mill practice, with the same bleach inan amount to supply 9.0 per cent of chlorine. The bleaching was continued for a total of 8 hours and 45 minutes, followed by washing.

The product bleached in accordance with standard mill practice had a brightness of 47.1 measured by a General Electric Company reiiectance meter. That bleached in accordance with this invention had a brightness of 50.1, thus showing that the practice of the present invention results in 3.0 per cent higher brightness as creased the yield by about '7 per cent over th regular hypochlorite process.

The amounts oi pulp. water, titanium compound; alkali and oxidizing bleaching agent may vary within wide limits, and the present invention is not limited to any particular ratio of ingred ients which will, in any event; depend in large part upon the particular pulp, bleaching agent, titanium compound, and alkali' used, as

well as upon the particular degree of brightness,

which is to be attained, as will be understood by those skilled in the 'art.' Thus, limits can not be stated for the various reagents which are applicable to all conditions. However, the practice of the invention is indicated and will be understood by those skilled in the art from the foregoing example.

As indicated above, the titanium compound need not necessarily be introduced in solution. For example, a suitable titanium compound in which the titanium has a valence less than four may be mixed with any of the ordinary commercial alkali silicates. Titanium hydroxide will thus be produced which will be maintained dispersed throughout the sodium silicate, and this dispersion may then be added to the suspension to supply both alkali and the necessary titanium compound.

Reference has been made herein and in the claims to conversion of the titanium in the titanium compound to the quadrivalent state. Some persons have thought that titanium may also exist in a hexavalent state. Whether or not that be true, it is clear'that it is converted at least to the quadrivalent form.

Other modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

According to the provisions of the patent statutes, I have explained the principle and method of practicing my invention and have described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended only after said chlorination action has been sub-= compared with standard established hy'pochlostantially completed.

2. In a method of bleaching cellulose pulp in an aqueous suspension containing a chlorinecontaining bleaching agent and in which bleaching is assisted by addition of a titanium compound in which the titanium has a valence less than four, the steps comprising exposing said pulp to the action of said bleaching agent until chlorlnating action of said agent is substantially completed, and then adding said titanium compound. v

3. In a method of bleaching cellulose pulp in an aqueous alkaline suspension containing a chlorine-containing bleaching agent and in which bleaching is assisted by addition of a titanium compound in which the titanium has a valence less than four, the steps comprising exposing said pulp to the action of said bleaching agent until chlorinating action oi saidagent is substantially completed, and then adding said titanium compound.

4. A method according to claim 3, said agent being a hypochlorite.

5. A method according to claim 3, said agent being a chlorite.

6. In a method of bleaching cellulose in anaqueous suspension containing an alkali and an oxidizing bleaching agent capable of exerting a chlorinating action on the cellulose and in which bleaching is assisted by addition oi a titanium compound in which the titanium has a valence less than four. the step comprising deferring addition of said alkali and titanium compound to the suspension until said chlorinating action has been substantially completed.

'7. A method according to claim 6, said bleaching agent being a hypochlorite.

8. A method according to claim 6, said bleaching agent being sodium hypochlorite.

9. A method according to claim 6, said bleaching agent being an alkaline earth hypochlorite.

10. A method according to claim 6, said titanium compound being titanous sulfate.

11. A method according to claim 6, said titanium compound being titanous sulfate. and said bleaching agent being a hypochlorite.

12. In a method of bleaching cellulose in an aqueous. alkaline suspension containing free oxygen and an oxidizing bleaching agent capable said cellulose with an oxidizing bleaching agent capable of exerting a chlorinating action upon the cellulose, agitating the suspension until said chlorinating action is substantially complete, and then adding alkali and a titanium compound in which the titanium has a valence less than four.

15. A method according to claim 14, said agent being a hypochlorite. i I p 16. A method according to claim 14, said bleaching agent being a hypochlorite and said suspension being substantially saturated with air whensaid titanium compound is added.

17. In a multi-stage process of bleaching cellulose in which bleaching is eiiected' initially by elemental chlorine and in a later stage by a chlorine-containing oxidizing bleaching agent, the improvement comprising adding oxygen and a titanium compound in whichthe titanium has a valence less than i'our to the cellulose furnish during said later stage after the chlorinating action upon the cellulose and impurities has been substantially ended.

. ALFRED O. BRAGG. 

